1. Field of the Invention
The present invention relates to the transmission technology of digital information, and more particularly relates to a technology suited for an amplitude shift keying (ASK) receiver for demodulating a signal modulated by an ASK method, using a digital signal which is encoded by a Manchester code, into its original signal.
2. Description of the Related Art
The principle of a general ASK receiver is described with reference to FIGS. 1 and 2. FIG. 1 shows the general configuration an ASK receiver. FIG. 2 shows an example of the operational waveform of each unit of the ASK receiver shown in FIG. 1.
A radio frequency (RF)-band or intermediate frequency (IF)-band signal (signal (A) in FIG. 2) is inputted to a logarithmic amplifier 101. This signal is obtained by applying amplitude modulation to a carrier wave by a digital signal encoded by a Manchester code. The logarithmic amplifier 101 logarithmically amplifies this inputted signal.
An envelope detector 102 applies envelope detection to the signal outputted from the logarithmic amplifier 101 and outputs the envelope of the signal (signal (B) in FIG. 2).
A duty correction circuit 103 reproduces its original digital signal (signal (C) in FIG. 2) from the envelope outputted from the envelope detector 102.
A demodulator 104 decodes the digital signal encoded by a Manchester code into its original digital data (data (D) in FIG. 2).
Here, the Manchester code is described.
When original data is “1”, the Manchester code encodes the former half and latter half of one prescribed cycle (Ts) into “0” and “1”, respectively. If the original data is “0”, the Manchester code encodes the former half and latter half of one prescribed cycle (Ts) into “1” and“0”, respectively. However, the Manchester code can also encode the former half and latter half of one prescribed cycle (Ts) into “1” and “0”, respectively. If the original data is “0”, the Manchester code can also encode the former half and latter half of one prescribed cycle (Ts) into “1” and “0”, respectively. In either case, the ideal ratio between a period during which “1” is outputted and a period during which “0” is outputted of the digital signal encoded by the Manchester code is 50:50.
Next, the duty correction circuit 103 shown in FIG. 1 is further described.
As described above, the duty correction circuit 103 reproduces a digital signal encoded by a Manchester code by the output of the envelope detector 102. It is ideal for the output of the duty correction circuit 103 to switch from “0” (low level) to “1” (high level) or from “1” (high level) to “0” (low level) at just the half time of one cycle (Ts). In this case, if the ratio between the time of “1” and time of “0” in one cycle (Ts) (duty ratio) somewhat deviates from the ideal 50:50, the demodulator 104 cannot determine its original data to fail to decode the digital signal. Therefore, it is necessary for the duty correction circuit 103 to prevent the duty ratio of a digital signal to reproduce from deviating from 50:50 (50%) as much as possible.
Here, FIG. 3 is described. FIG. 3 shows an example of a general circuit configuration of the duty correction circuit 103.
The circuit shown in FIG. 3 comprises a fairly higher-order filter (low-pass filter or band-pass filter) 111, a low-pass filter (LPF) composed of a resistor 112 and a capacitor 113, for extracting a low frequency component from the output of this filter 111 and a comparator 114 to the positive and negative input terminals of which the output of the filter 111 and the output of the LPF, respectively, are inputted.
The comparator 114 compares the voltage of a signal outputted from the filter 111 with the average voltage of the signal, obtained by the LPF and outputs this comparison result. In this case, if the duty ratio of the signal outputted from the filter 111 is 50%, the average voltage of the signal is located at just the center of the amplitude width of the signal. Therefore, by the comparator 114 determining the height of the voltage of the signal using this average voltage as its threshold, the original digital signal encoded by a Manchester code is reproduced.
Japanese Patent Application No. 2001-211214 discloses the circuit shown in FIG. 4 as the duty correction circuit 103. This circuit comprises a comparator 121 and an integrator 122.
The integrator 122 comprises a resistor 123, a capacitor 124 and an operational amplifier 125. In this case, the capacitor 124 is inserted between the inversion input terminal and output terminal of the operational amplifier 125, and one terminal of the resistor 123 is connected to the inversion input terminal of the operational amplifier 125. To the other terminal of this resistor 123, the voltage of a signal inputted to the integrator 122 is applied. To the non-inversion input terminal of the operational amplifier 125, reference voltage Vref2 (for example, voltage of approximately a half of the power voltage) is applied.
Voltage Vref1 outputted by the operational amplifier 125 is the output of the integrator 122. This voltage Vref1 is inputted to one comparison terminal (positive input terminal) of the comparator 121. To the input terminal of the integrator 122, the output signal of the comparator 121, that is, the original digital signal encoded by a Manchester code, which is the output of the circuit shown in FIG. 4, is inputted.
An envelope signal outputted by the envelope detector 102 is inputted to the other comparator terminal (negative input terminal) of the comparator 121.
In the circuit shown in FIG. 4, if the duty ratio of the digital signal that is encoded by a Manchester code and is outputted by the comparator 121 deviates from 50%, the output voltage Vref1 of the integrator 122 changes in such a way as to bring this duty ratio close to 50%. Thus, the duty ratio of the output signal of the circuit shown in FIG. 4 is improved regardless of the waveform of the envelope signal outputted by the envelope detector 102.
In order to the circuit shown in FIG. 3 to operate well, it is preferable for the duty ratio of the signal outputted from the filter 111 to be close to 50%. For that purpose, the filter 111 must be higher-order. However, in order to build a higher-order filter, a lot of parts are needed to lead to cost-up and also make its integration difficult.
However, the circuit shown in FIG. 4 requires no higher-order filter. However, if the data rate of the original digital data is high (one cycle (Ts) is short), in order to reproduce the digital signal by this circuit, a higher-speed operational amplifier 125 is also needed to increase current consumption. Furthermore, if the reference voltage Vref2 of the integrator 122 deviates from a prescribed voltage, the duty ratio deviates. If the envelope signal inputted to the circuit shown in FIG. 4 rapidly changes, it takes much time in connection with the time constant of the integrator 122 to obtain the stable duty ratio of the digital signal. Therefore, if the time constant of the integrator 122 is increased to stabilize the duty ratio of the digital signal to reproduce, the responsiveness on the rapid change of an input signal degrades.